In common to all such burners is the demand by environmental authorities for increasingly low emissions of primarily NOx. The key issue for obtaining low emissions of NOx and other pollutants is to obtain a sufficient distribution and evaporation of said liquid fuel in the combustion process avoiding hot spots resulting in higher emissions of NOx as well as spots with combustion at too low temperatures resulting in high emissions of CO. The main task of an arrangement for preparation of a fuel as defined in the introduction is therefore to provide a sufficiently uniform distribution of the fuel by evaporation thereof before the fuel is mixed with air/oxidant. However, in some cases a “defined” or “controlled” non-uniform distribution may also be accepted where a NOx/turndown trade-off would be a possible solution.
Different ways have so far been chosen for obtaining this. Water or steam has been injected in combination with the liquid fuel in particular to limit the flame temperature and as a secondary effect to produce a more refined spray. Another way to proceed has been to utilize a higher feed pressure of the fuel obtaining a well atomized spray and increasing the time for mixing/evaporation from the injection point to combustion. A third route has been to convert the liquid fuel to a gas in a separate reactor using steam or combustion in oxygen poor environments.
The alternative last mentioned appears to be the most attractive one from the point of view of obtaining low emissions of NOx, since it is easier to obtain a homogenous mixture of air and fuel when starting the mixing process by mixing air and a gas as fuel than air and atomized liquid fuel to be evaporated. The hot gas fuel can be better distributed throughout the entire amount of air, so that lower temperature combustion results and by that lower emissions of NOx. Moreover, possible combustion of not-evaporated and/or unmixed liquid fuel will also result in higher emissions of NOx.
Russian patent document 2106574 discloses a said arrangement in which a pipe containing liquid fuel is exposed to hot combustion gases in a space in which the flame of the burner is located evaporating the fuel inside the pipe. The hot gas fuel is then mixed with air and conveyed into said space for combustion. A disadvantage of this arrangement is that as a consequence of the thin pipe walls, very hot zones of the pipe will result, which involves a risk of coking of the fuel producing deposits on the internal walls of the pipe. Furthermore, the pipe is unprotected and exposed to corrosion and wear, so that the lifetime thereof will be limited. There is also risk of occurrence of vibrations of the pipe extending freely in said space. Furthermore, it is difficult to control the heating and evaporation of the liquid fuel, since the thin walls of the pipe are very sensitive to changes of the operation conditions of the burner.